The CDPD system enables wireless transmission of data over existing cellular systems, such as the Advanced Mobile Phone System (AMPS). The CDPD system was designed to provide data communications in the cellular frequency range without impeding voice communications. The system uses "mobile" telephone channels just as a modem uses a telephone line but it is designed to jump from one frequency to another when necessary, i.e., when a new telephone call starts in the cell.
The first live demonstrations of CDPD took place in early 1993. Advocates for CDPD believe the technology will be inexpensive and cost effective because it builds on top of existing cellular infrastructure and does not require any additional spectrum allocation. Instead of requiring additional frequencies for data communications, CDPD uses temporal "gaps" that occur between voice calls to send data in bursts. (A voice call involves the period of time between the dialing of a number or reception of a call to hang-up of the handset on either terminal unit.) Technology has been developed to "frequency hop" and seek out gaps in cellular conversations in the cellular spectrum. The frequency-hopping technology is expected to be sufficiently robust to handle the designed data transfer rate (19.2 kbps) in crowded cellular markets.
In addition, new personal wireless communications products have been, and are being, designed to employ CDPD technology in combining all-purpose mobile communications with the technology of cellular phones, fax machines, modems, electronic mail, and pen computing. Such products will provide expanded communication capabilities to users.
Therefore, using existing hardware technology as a building block, CDPD will allow data to be transmitted across existing cellular networks, enabling idle voice time on cellular systems to be filled with packets of data. CDPD technology, with a data transmission rate of 19.2 kbps, is up to four times faster than competing wireless services. However, one of the requirements of a CDPD system is the capability of a mobile unit or base station unit to efficiently (i.e., quickly and reliably) identify a CDPD transmission.